In an IC test apparatus having a plurality of test signal supply paths (hereinafter "channel") which provide test pattern signals to a plurality of pins of the IC to be tested, delay times among the channels should be matched. Thus, each delay time per each channel has to be adjusted to be same delay time and such adjusted delay time should be maintained for a long period of time. However, if IC circuits are employed to form each channel to decrease the size of the test apparatus, there arises a problem in that, in such IC circuits, delay times for propagation of signals vary depending on temperature changes.
Particularly, if the ICs with CMOS (complementary metal-oxide semiconductor) structure are used, such a change in the delay time conspicuous since the heat generation is completely different whether it is in rest or in operation. Namely, in the CMOS circuit, in a non-operational state (when no signals are generated therefrom), the amount of a current consumed therein is very small. In contrast, if the CMOS ICs are in an operational state (when a logic of the signal is inverted), they consume the current several thousand times as much as they do in the non-operational state. Thus, in the ICs with the CMOS structure, the amounts of the currents consumed therein are largely different between the operational and non-operational states. Accordingly, self-heating values in each state are different. If the operational and non-operational states are irregularly happen, the temperature of the IC chips increases only when they are in the operational state, which is impossible to control. As a result, mismatches of the delay times are caused between the channels.
In order to solve the above problem, the conventional device has employed ICs having temperature compensatory function for a circuit aiming to maintain the delay times in a certain period.
FIG. 9 shows a structure of the conventional IC having a temperature compensatory function therein. In FIG. 9, reference numeral 10 is a chip forming an IC. The chip 10 consists of a target circuit 11 to form a channel of the IC test apparatus and its delay time is to be maintained in a certain period, a temperature sensor 12 formed in a vicinity of the target circuit 11, a plurality of heater elements H dispersed in a vicinity of the target circuit 11, and a plurality of switch elements 13 for turning on/off a current applied to the heater elements H. In the conventional device, detecting signals from the temperature sensor 12 are given to a heater control device 20, control output signals from the heater control device 20 are given to the switch elements 13 wherein the current applied to the heater H is turned on/off so as to consistently maintain the temperature inside the chip 10.
Namely, when the target circuit 11 is in the non-operational state, the heater elements H is heated by a current (which is the same current as consumed in the target circuit 11) applied thereto. Then, an operation of the target circuit 11 begins so as to turn off the current applied to the heater elements H when a surrounding temperature of the target circuit 11 increases.
When a signal is given to the target circuit 11 and its operation begins, the temperature of the circuit increases. Then, the temperature sensor 12 detects the temperature change and shuts off the current applying to the heater elements H, causing the time delay. However, this conventional structure has a disadvantage in that: since the detection of the temperature change is subject to the temporary increase of the temperature inside the chip 10, it causes a fluctuation in the delay times.
Further, because of the delay in the detection, the currents are applied to both of the target circuit 11 and heater elements H. This means that an electric power source receives about twice as large current as the current consumed in the target circuit 11. Therefore, material used for the electric power source should endure at least twice as large current as the current consumed by the target circuit 11. The number of the electric power sources correspond to the number of the circuits 11. For example, in an IC test device, 100 electric power sources may be needed to match the number of the IC pins, that is 100 IC pins, resulting the increase of the manufacturing costs.
Another conventional method contains a circuit which controls the propagation delay time in an IC by controlling a heater in the IC and the amount of heat given to the IC by the heater. Namely, the conventional method realizes a control circuit for adjusting the signal propagation delay time in substantially constant time by controlling the temperature of the IC.
According to the conventional method, the propagation delay time of a signal which passes at least one part of the IC can be measured and compared with a standard delay time. If the measured delay time is shorter than the standard delay time, it is controlled to increase the heat generation of the heater. On the other hand, if the measured delay time is longer than the standard delay time, it is controlled to decrease the heat generation from the heater.
FIG. 10 is a block diagram showing a conventional structure disclosed in the Japanese Patent Laying-Open No. 1-114067. Since a delay measuring circuit 30 is included in an IC chip 32, a delay time in the delay measuring circuit 30 is affected by the temperature as substantially the same as other circuit in the chip IC 32.
A heater 34 thermally connected to the IC chip adjusts the signal transmission delay time by selectively heating the IC chip 32. The heater 34 is preferably an integrated heating element which is mounted on the IC chip 32 with other circuit components. The heater 34 is usually provided relatively near the delay measuring circuit 30. Thus, it is able to minimize the time delay until the delay measuring circuit 30 is heated by the heater 34. A typical IC chip using a metallic lead frame has a good thermal conductivity. Thus, it is able to transmit the heat generated by the heater 34 to each circuit inside the chip. Usually, a plastic or ceramic package 36 having a relatively good thermal insurability surrounds the chip 32. With the package 36, the inside of the IC chip can keep a higher temperature than an ambient temperature.
As described in the above, all circuits in the IC chip 32 are arranged to be in a vicinity each other and made of materials having high thermal conductivity. Therefore, all of the circuits inside the IC chip 32 are maintained in the uniform temperature. Further, factors changing the delay time in one circuit in the chip, such as the temperature and the voltage, are set to substantially equal in every circuit within the same IC chip. Therefore, it is possible to adjust the delay time of all circuits in the chip by measuring either one of the circuits, such as the delay measuring circuit 30, and adjusting the temperature in the chip based on its information.
A control means 40 controls the heater 34 based on the measured result of the delay time received from the delay measuring circuit 30. Namely, if a measured value of the delay time is shorter than a desired time, the control means 40 controls the heater 34 to increase the heat generation. In contrast, if a measured value of the delay time is longer than the desired time, the control means 40 controls the heater 34 to decrease the heat generation. Accordingly, the delay time is precisely controlled by this procedure.
A standard delay circuit 42 in FIG. 10 generates standard delay signals corresponding to a desired transmission delay time. A delay comparison circuit 44 compares a standard delay signal from the standard delay circuit 42 and a measured delay signal from the delay measuring circuit 30. The output of the delay comparison circuit 44 is indicative of a time difference between the desired delay time and the delay time which is actually measured. Then, the heater 34 is controlled by a heater control signal so as to decide whether the generated heat is maintained or adjusted, based on a relationship between the standard delay signal and the measured delay signal. A standard delay time setting circuit 50 including a micro processor sets up a desired standard delay time. Further, the delay measuring circuit 30 measures the transmission delay time in response to a test signal received from a test signal source 52.
According to the foregoing method, the temperature inside the IC chip can be substantially maintained. However, it is impossible for the conventional method to handle local temperature changes in a short period of time.